The invention generally relates to data communication systems and more specifically relates to data modems adapted for two-wire/half-duplex operation through direct distance dialed (DDD) networks.
The use of data modems to transmit data over ordinary telephone lines through direct distance dialed (DDD) networks is commonplace today. Presently, independent manufacturers of data modems supply a large portion of such modem equipment as compared to the past wherein telephone companies supplied virtually all the modem equipment. In today's marketplace, however, the independent modem manufacturers are still faced with the problem of establishing data connections through direct distance dialed telephone lines and the associated equipment provided by telephone companies. Accordingly, much ingenuity is exercised by the independent manufacturers of modems in adapting their products to provide maximum efficiency while operating the telephone equipment over which they have no control.
As an example, the independent manufacturers of modems must adapt their products to operate over long distance voice communication telephone networks. In such telephone networks, the telephone company introduces large amounts of amplification in a pair of separate two-wire/one-way paths located in the DDD networks. Conventional two-to-four wire hybrid circuits convert between a two-wire/two-way circuit found at a location including a subscriber's telephone, or modem, and the pair of two-wire/one-way circuits normally found in the DDD networks between central offices. If these hybrid circuits were perfectly balanced, no signal intended to be transmitted from a sending to a receiving line would reflect back into the input of the sending line. In actual practice, however, the numerous switching connections and variety of trunks and subscriber lines make it impossible to have anywhere near perfect balance in the hybrid circuits. As a result, a portion of the signal always loops around the pair of two-wire/one-way paths and is reflected back to the two-wire/two-way circuits at the subscriber locations. These reflected signals are termed "echo signals".
In order to avoid these undesirable echoes, DDD networks include echo suppressors. In normal operation, an echo suppressor provides a low impedance to a voice signal traveling in direction on one two-wire/one-path while simultaneously providing a high impedance in the other two-wire/one-way path of the network so that echoes of the voice signals are blocked.
When bi-directional data transmission is desired, these echo suppressors must be inactivated. This is accomplished by transmitting a tone of a particular frequency over the established direct distance dialed telephone line which is connected between two subscriber locations. The echo suppressors respond to this unique tone by assuming a low impedance in both directions, wherein the echo suppressors are effectively removed from the direct distance dialed network.
The assignee of this invention has provided a dramatic increase in the data throughput of data through a direct distance dialed network when modems are operating in a two-wire/half-duplex mode. The invention assigned to the same assignee as the present invention has been termed FASTAR which is a trademark of the assignee of this invention. The FASTAR invention is fully described and claimed in U.S. Pat. No. 3,783,194 issued Jan. 1, 1974.
Briefly, the FASTAR invention generates a unique tone to initially disable the echo suppressors. Thereafter, whenever the network is free of data transmission in either direction, and the line must remain in a data transmission mode, a tone generator in a modem is enabled. The tone generator supplies a signal at a frequency which is outside the frequency range of data transmission, and it assures that energy is not absent from the network for a predetermined time which would cause the echo suppressors to again be enabled. This additional signal thus maintains the echo suppressors in a disabled condition during absence of data transfer in either direction. As a result of the FASTAR invention, data transmission is accomplished with reduced turn-around time, i.e. one modem can send data almost immediately after the other modem finished sending a block of data.
Although dramatically improving data throughput, the FASTAR invention does require an operator to monitor the status of the modem and make a presumption that the echo suppressors have been disabled. Our invention not only obviates the requirement for an operator's attention to the modem, but in addition verifies that a line is established between communicating modems. Furthermore, our invention automatically verifies that the echo suppressors are disabled and an acceptable line for data communication exists between the modems.